BACKGROUND OF THE INVENTION

[0001]
1. Field of the Invention

[0002]
The present invention relates to a high speed amplifier and a comparator using the same applied to a sequential successive approximation analogtodigital (A/D) converter.

[0003]
2. Description of the Related Art

[0004]
Generally, A/D converters are required to accurately convert an analog signal into a digital signal at high speed.

[0005]
One typical A/D converter is a sequential successive approximation A/D converter which is constructed by a comparator for comparing an input voltage with a reference voltage derived from an input analog voltage, a sequential approximation register (SAR), a digitaltoanalog (D/A) converter for performing a D/A conversion upon the content of the sequential approximation register to generate the input voltage, and a control circuit for controlling the content of the sequential approximation register in accordance with the output signal of the comparator. This will be explained later in detail.

[0006]
A first prior art comparator is constructed by a plurality of cascaded fast amplifiers of a twoinput and twooutput type with a low gain (amplification) and a low output impedance operable at high speed, and a slow amplifier of a twoinput and oneoutput type with a high gain (amplification) and a high output impedance operable at low speed (see: FIG. 7 of JP10200385A). This also will be explained later in detail.

[0007]
In the abovedescribed first prior art comparator, however, when the input voltage crosses the reference voltage, one return delay time is caused by each of the fast amplifiers, so that the total delay time would be increased. Also, the operation speed of the fast amplifiers per se cannot be increased. Thus, the first prior art comparator cannot be operated at a high speed.

[0008]
A second prior art comparator, switches are provided at the output ends of each of the fast amplifiers of the first prior art comparator (see: FIGS. 1 and 2 of JP10200385A). As a result, every time a stable time period has passed after one comparison operation, the abovementioned switches are turned ON, so that the output voltages of the fast amplifiers are initialized or reset to their operating points. Therefore, since no return delay times are generated, the total delay time is not increased. Thus, the second prior art comparator may be operated at high speed. This also will be explained later in detail.
SUMMARY OF THE INVENTION

[0009]
In the abovedescribed second prior art comparator, however, since the OFF timings of the abovementioned switches must be determined in view of the maximum values of delay time periods of the fast amplifiers, the stable time period is not always short, so that the total delay time would not always be decreased. Also, the operation speed of the fast amplifiers per se cannot be increased. Thus, the second prior art comparator would not always be operated at high speed.

[0010]
According to the present invention, in an amplifier including first and second power supply terminals, first and second output terminals, a first load connected between the first power supply terminal and the first output terminal, a second load connected between the first power supply terminal and the second output terminal, a constant current source connected to the second power supply terminal, a first transistor connected between the first output terminal and the constant current source, a control terminal of the first transistor being adapted to receive an input voltage, and a second transistor connected between the second output terminal and the constant current source, a control terminal of the second transistor being adapted to receive an reference voltage, an amplification and output impedance switching circuit is connected between the first and second output terminals, so that the amplification and output impedance switching circuit controls the amplification and output impedance of the amplifier in accordance with a control signal.

[0011]
Also, first, the control signal is a first value to make the amplifier have a first amplification and a first output impedance, and then, the control signal is a second value to make the amplifier have a second amplification higher than the first amplification and a second output impedance higher than the first output impedance. Thus, the operation speed of the amplifier is substantially increased.

[0012]
Thus, in the amplifier according to the present invention, the amplification and output impedance is controlled by changing the resistance between the first and second output terminals without changing the resistance values of the first and second loads. In other words, the operating output points of the amplifier are unchanged at a switching of the amplification and output impedance by the control signal.

[0013]
Further in a comparator including a plurality of first amplifiers of a twoinput and twooutput type connected in series and a second amplifier of a twoinput and oneoutput type connected to a final one of the first amplifiers, the abovementioned amplifier is used as at least one of the first amplifiers, so that the total delay time caused by the first amplifiers can be decreased.

[0014]
Thus, even in the comparator according to the present invention, even when the amplification and output impedance of the abovementioned amplifier is changed, the operating output points of the amplifier are unchanged so that the delay of comparison operations is not generated.
BRIEF DESCRIPTION OF THE DRAWINGS

[0015]
The present invention will be more clearly understood from the description set forth below, as compared with the prior art, with reference to the accompanying drawings, wherein:

[0016]
FIG. 1 is a circuit diagram illustrating a prior art sequential successive approximation A/D converter;

[0017]
FIG. 2 is a table showing a digital output signal of the sequential successive approximation A/D converter of FIG. 1;

[0018]
FIG. 3 is a circuit diagram of a first prior art comparator;

[0019]
FIG. 4 is a detailed circuit diagram of the fast differential amplifier of FIG. 3;

[0020]
FIG. 5 is a timing diagram for explaining the operation of the comparator of FIG. 3;

[0021]
FIG. 6 is a circuit diagram of a second prior art comparator;

[0022]
FIG. 7 is a timing diagram for explaining the operation of the comparator of FIG. 6;

[0023]
FIG. 8 is a circuit diagram of a comparator including an embodiment of the fast/super fast amplifier according to the present invention;

[0024]
FIG. 9 is a detailed circuit diagram of a first example of the fast/super fast differential amplifier of FIG. 8;

[0025]
FIG. 10 is a timing diagram for explaining the operation of the comparator of FIG. 8;

[0026]
FIG. 11 is a detailed circuit diagram of a second example of the fast/super fast differential amplifier of FIG. 8;

[0027]
FIG. 12 is a detailed circuit diagram of a third example of the fast/super fast differential amplifier of FIG. 8;

[0028]
FIGS. 13, 14 and 15 are detailed circuit diagrams illustrating modifications of the fast/super fast differential amplifier of FIGS. 9, 11 and 12, respectively; and

[0029]
FIG. 16 is a circuit diagram illustrating a modification of the comparator of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030]
Before the description of the preferred embodiment, a prior art sequential successive approximation A/D converter and prior art comparators applied to the sequential successive approximation A/D converter will be explained with reference to FIGS. 1 to 7.

[0031]
In FIG. 1, which illustrates a prior art sequential successive approximation A/D converter, a comparator 1 compares an input voltage V_{in }with a reference voltage V_{ref }to generate an output voltage V_{out}. A control circuit receives the output signal of the comparator 1 to control a content of a sequential approximation register (SAR) 3. A digitaltoanalog (D/A) converter 4 performs a D/A conversion upon the content of the sequential approximation register 3 to generate the input voltage V_{in}.

[0032]
On the other hand, an end of an input capacitor 5 is connected to a positive input of the comparator 1, and an end of a reference capacitor 6 is connected to a negative input of the comparator 1. In this case, the capacitance of the input capacitor 5 is the same as that of the reference capacitor 6, so that the input capacitor 5 and the reference capacitor 6 form a complete differential pair.

[0033]
A switch S1 is connected to the other end of the input capacitor 5 to receive an analog voltage AIN.

[0034]
A switch S2 is connected between the output of the D/A converter 4 and the other end of the input capacitor 5 to receive the analog output voltage of the D/A converter 4.

[0035]
Switches S3 and S4 are connected between the other end of the reference capacitor 6 and a reference line REF which is further connected via switches sfa and sfb to the positive input and negative input, respectively, of the comparator 1.

[0036]
The switches S1 and S2 are complementary. That is, when one of the switches S1 and S2 is turned ON, the other is turned OFF. Also, the switches S3 and S4 are complementary. That is, when one of the switches S3 and S4 is turned ON, the other is turned OFF.

[0037]
Since the two switches S1 and S2 are provided for the input capacitor 5 while the switches S3 and S4 are provided for the reference capacitor 6, the capacitance of the Input capacitor 5 including its parasitic capacitance is substantially the same as that of the reference capacitor 6 including its parasitic capacitance.

[0038]
On the other hand, the switches S1 and S3 are complementary. That is, when one of the switches S1 and S3 is turned ON, the other is turned OFF. Also, the switches S2 and S4 are complementary. That is, when one of the switches S2 and S4 is turned ON, the other is turned OFF. Thus, the noise due to the switching of the switches S1, S2, S3 and S4 can be diminished.

[0039]
The control circuit 2 also controls the switches S1 and S4, the switches S2 and S3, and the switches sfa and sfb.

[0040]
The operation of the sequential successive approximation A/D converter of FIG. 1 includes a sample/hold mode for sampling and holding the analog voltage AIN as a reference voltage V_{ref}, and a comparison mode. In the comparison mode, the comparator 1 carries out n times of comparison operations to obtain an nbit digital signal such as a 4bit digital signal denoted by D3, D2, D1 and D0.

[0041]
First, in the sample/hold mode, the control circuit 2 turns ON the switches S1, S4, sfa and sfb, and turns OFF the switches S2 and S3. As a result, the Input capacitor 5 is charged by the difference between the analog voltage AIN and the voltage at the reference line REF, while the ends of the reference capacitor 6 are not charged. Therefore, the reference voltage V_{ref }at the reference line REF, i.e., at the negative input of the comparator 1 corresponds to the analog voltage AIN.

[0042]
In a comparison mode, the control circuit 2 turns ON the switches S2 and S3 and turns OFF the switches S1, S4, sfa and sfb, so that comparison operations are carried out four times. Note that the reference voltage V_{ref }(AIN) is maintained at the negative input of the comparator 1 by the turnedOFF switches sfa and sfb. Also, assume that, when the sequential approximation register 3 generates a 4bit digital signal (D3, D2, D1, D0), the digitaltoanalog converter 4 generates the input voltage V_{in }as illustrated in FIG. 2. In this case, assume that the analog input voltage AIN is between V_{5 }and V_{6}.

[0043]
First, the control circuit 2 sets “1000” where only the most significant bit (MSB) D3 is “1” in the sequential approximation register 3. As a result, the digitaltoanalog converter 4 supplies V_{8 }to the capacitor 5, so that V_{in}=V_{8}−AIN+V_{ref}. Therefore, since V_{in}=V_{8}−AIN+V_{ref}>V_{ref}) the output voltage V_{out }of the comparator 1 is high.

[0044]
Next, upon receipt of the high output voltage V_{out }of the comparator 1, the control circuit 2 resets the most significant bit D3 (D3=0) and sets the second significant bit D2 (D2=1). That is, the control circuit 2 sets “0100” in the sequential approximation register 3. As a result, the digitaltoanalog converter 4 supplies V_{4 }to the capacitor 5, so that V_{in}=V_{4}−AIN+V_{ref}. Therefore, since V_{in}=V_{4}−AIN+V_{ref}<V_{ref}, the output voltage V_{out }of the comparator 1 is low.

[0045]
Next, upon receipt of the low output voltage V_{out }of the comparator 1, the control circuit 2 sets the third significant bit D1 (D1=1). That is, the control circuit 2 sets “0110” in the sequential approximation register 3. As a result, the digitaltoanalog converter 4 supplies V_{6 }to the capacitor 5, so that V_{in}=V_{6}−AIN+V_{ref}. Therefore, since V_{in}=V_{6}−AIN+V_{ref}>V_{ref}) the output voltage V_{out }of the comparator 1 is high.

[0046]
Finally, upon receipt of the high output voltage V_{out }of the comparator 1, the control circuit 2 resets the third significant bit D1 (D1=0) and sets the least significant bit (LSB) D0 (D0=1). That is, the control circuit 2 sets “0101” in the sequential approximation register 3. As a result, the digitaltoanalog converter 4 supplies V_{5 }to the capacitor 5, so that V_{in}=V_{5}−AIN+V_{ref}. Therefore, since V_{in}=V_{5}−AIN+V_{ref}<V_{ref}, the output voltage V_{out }of the comparator 1 is low.

[0047]
Thus, the 4bit digital signal (D3, D2, D1, D0)−(0, 1, 0, 1) is obtained.

[0048]
Note that a time period of comparison operation for 1bit is defined by T_{cycle}.

[0049]
In the sequential successive approximation A/D converter of FIG. 1, in order to carry out an A/D conversion, the comparator 1 is required to accurately amplify a very small difference between the input voltage V_{in }and the reference voltage V_{ref }at a high speed. For this purpose, the comparator 1 is generally formed by multistage cascaded amplifiers (or differential amplifiers) each with a small gain (amplification) and a small output impedance.

[0050]
In FIG. 3, which illustrates a first prior art comparator applied to the sequential successive approximation A/D converter of FIG. 1 (see: FIG. 7 of JP10200385A), differential amplifiers 101, 102, 103 and 104 are cascaded, i.e., connected in series by capacitors 111 a, 111 b, 112 a, 112 b, 113 a and 113 b. The differential amplifiers 101, 102 and 103 are fast ones of a twoinput and twooutput type with a low gain (amplification) and a low output impedance operable at a high speed, while the differential amplifier 104 is a slow one of a twoinput and oneoutput type with a high gain (amplification) and a high output impedance operable at a low speed. Thus, the entirety of the differential amplifiers 101, 102, 103 and 104 can realize a high gain operable at a high speed.

[0051]
In the fast differential amplifiers 101, 102 and 103, the relationship between the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a and the output voltages V_{c} 1 b, V_{c} 2 b and V_{c} 3 b is opposite in phase to the relationship between the input voltages V_{1} 1 a, V_{1} 2 a and V_{c} 3 a and the input voltages V_{1} 1 b, V_{1} 2 b and V_{1} 3 b. Also, in the slow differential amplifier 104, when V_{1} 4 a>V_{1} 4 b, the output voltage V_{out }is high (=V_{DD}), and when V_{1} 4 a≦V_{1} 4 b, the output voltage V_{out }is low (=GND).

[0052]
The input ends of the differential amplifiers 101, 102, 103 and 104 are connected via switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b to a reference line REF. The switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b are controlled by a control circuit 120 so that the operating points of the input voltages of the differential amplifiers 101, 102, 103 and 104 can be at the reference voltage V_{ref }before the comparison operations.

[0053]
In FIG. 4, which is a detailed circuit diagram of each of the fast differential amplifiers 101, 102 and 103 of FIG. 3, the fast differential amplifier 101 (102, 103) includes a differential pair formed by two nchannel MOS transistors Q_{n1 }and Q_{n2}, a constant current source formed by an nchannel MOS transistor Q_{n3 }with a definite gate voltage V_{G }connected between a common source of the nchannel MOS transistors Q_{n1 }and Q_{n2 }and the ground terminal GND, and loads formed by sourcetogate connected pchannel MOS transistors Q_{p1 }and Q_{p2 }connected between the drains of the nchannel MOS transistors Q_{n1 }and Q_{n2 }and a power supply terminal V_{DD}. In this case, gates of the nchannel MOS transistors Q_{n1 }and Q_{n2 }receive the input voltages V_{1} 1 a (V_{1} 2 a, V_{1} 3 a) and V_{1} 1 b (V_{1} 2 b, V_{1} 3 b), while drains of the nchannel MOS transistors Q_{n1 }and Q_{n2 }or output terminals OUTa and OUTb generate the output voltages V_{c} 1 a (V_{c} 2 a, V_{c} 3 a) and V_{c} 1 b (V_{c} 2 b, V_{c} 3 b).

[0054]
In the fast differential amplifier of FIG. 4, the gain A_{v }and the output voltage V_{c }(such as V_{c} 1 a) can be represented by
A _{v} =g _{m} ·r
V _{c} =I _{out} ·r

[0055]
where r is an output impedance at the output terminal OUTa;

[0056]
g_{m }is the mutual conductance of each of the transistors Q_{n1 }and Q_{n2}; and

[0057]
I_{out }is a current flowing through the transistor Q_{n1 }or Q_{n2}.

[0058]
Note that the differential amplifier 104 can be constructed by a single and differential amplifier with a high gain (amplification) and a high output impedance at a low speed.

[0059]
The operation of the comparator of FIG. 3 is explained next with reference to FIG. 5. Even in this case, assume that the analog input voltage AIN is between V_{5 }(0101) and V_{6 }(0110) as shown in FIG. 2, and comparison operations are carried out four times with the value of the sequential approximation register 3 of FIG. 1 being 1000, 0100, 0110 and 1010.

[0060]
Before time t0, the control circuit 120 turns ON all the switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b, so that the input voltages V_{1} 1 a, V_{1} 1 b, V_{1} 2 a, V_{1} 2 b, V_{1} 3 a, V_{1} 3 b, V_{1} 4 a and V_{1} 4 b of the differential amplifiers 101, 102, 103 and 104 are initialized to V_{ref }(operating point).

[0061]
At time t0, the control circuit 120 turns ON all the switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b, so that the input voltages V_{1} 1 a, V_{1} 2 a, V_{1} 3 a, and V_{1} 4 a are separated from the input voltages V_{1} 1 b, V_{1} 2 b, V_{1} 3 b, and V_{1} 4 b, respectively.

[0062]
Also, at time t0, the input voltage V_{in }is caused to be V_{8 }(1000), so that the output voltages V_{c} 1 a, V_{c} 2 a, V_{c} 3 a and V_{c} 4 a of the differential amplifiers 101, 102, 103 and 104 are changed with delay time periods T1, T2, T3 and T4, respectively, determined by time constants determined by the capacitors 111 a, 111 b, 112 a, 112 b, 113 a and 113 b, the parasitic capacities and the like of the differential amplifiers 101, 102, 103 and 104. Therefore, in a time period T_{cycle }(1), the total delay time T is
T=T1+T2+T3+T4.

[0063]
Next, at time t1 after T_{cycle }(1) has passed, the input voltage V_{in }is switched from V_{8 }(1000) to V_{4 }(0100), so that the Input voltage V_{in }crosses the reference voltage V_{ref}. In this case, it will take return delay times ΔT1, ΔT2 and ΔT3 determined by the abovementioned time constants for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a to return to V_{ref }(operating point). Therefore, in a time period T_{cycle }(2), the total delay time period T is
T=T1+T2+T3+T4+ΔT1+ΔT2+ΔT3.

[0064]
Next, at time t2 after T_{cycle }(2) has passed, the input voltage V_{in }is switched from V_{4 }(0100) to V_{6 }(0110), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Even in this case, it will take return delay times ΔT1, ΔT2 and ΔT3 determined by the abovementioned time constants for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a to return to V_{ref }(operating point). Therefore, in a time period T_{cycle }(3), the total delay time period T is
T=T1+T2+T3+T4+ΔT1+ΔT2+ΔT3.

[0065]
Finally, at time t3 after T_{cycle }(3) has passed, the input voltage V_{in }is switched from V_{6 }(0110) to V_{5 }(0101), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Even in this case, it will take return delay times ΔT1, ΔT2 and ΔT3 determined by the abovementioned time constants for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a to return to V_{ref }(operating point). Therefore, in a time period T_{cycle }(4), the total delay time period T is
T=T1+T2+T3+T4+ΔT1+ΔT2+ΔT3.

[0066]
Thus, in the comparator of FIG. 3, the total delay time is increased by the return delay times ΔT1, ΔT2 and ΔT3 caused when the input voltage V_{in }crosses the reference voltage V_{ref}. Also, since the operation speed of the fast differential amplifiers 101, 102 and 103 per se cannot be increased, the operation speed of the comparator cannot be increased.

[0067]
In FIG. 6, which illustrates a second prior art comparator applied to the sequential successive approximation A/D converter of FIG. 1 (see; FIGS. 1 and 2 of JP10200385A), switches ss1, ss2 and ss3 are added to the output ends of the fast differential amplifiers 101, 102 and 103, respectively, of FIG. 3. The switches ss1, ss2 and ss3 are also controlled by the control circuit 120.

[0068]
The operation of the comparator of FIG. 6 is explained next with reference to FIG. 7. Even in this case, assume that the analog input voltage AIN is between V_{5 }(0101) and V_{6 }(0110) as shown in FIG. 2, and comparison operations are carried out four times with the value of the sequential approximation register 3 of FIG. 1 being 1000, 0100, 0110 and 1010.

[0069]
Before time t0, the control circuit 120 turns ON all the switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a, sf4 b, ss1, ss2 and ss3 so that the input voltages V_{1} 1 a, V_{1} 1 b, V_{1} 2 a, V_{1} 2 b, V_{1} 3 a, V_{1} 3 b, V_{1} 4 a and V_{1} 4 b of the differential amplifiers 101, 102, 103 and 104 are initialized to V_{ref }(operating point), and also, the output voltages V_{c} 1 a, V_{c} 1 b, V_{c} 2 a, V_{c} 2 b, V_{c} 3 a and V_{c} 3 b of the differential amplifiers 101, 102 and 103 are initialized to their operating points.

[0070]
At time t0, the control circuit 120 turns ON all the switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a, sf4 b, ss1, ss2 and ss3, so that the input voltages V_{1} 1 a, V_{1} 2 a, V_{1} 3 a, and V_{1} 4 a are separated from the input voltages V_{1} 1 b, V_{1} 2 b, V_{1} 3 b, and V_{1} 4 b, respectively.

[0071]
Also, at time to, the input voltage V_{in }is caused to be V_{8 }(1000), so that the output voltages V_{c} 1 a, V_{c} 2 a, V_{c} 3 a and V_{c} 4 a of the differential amplifiers 101, 102, 103 and 104 are changed with delay time periods T1, T2, T3 and T4, respectively, determined by time constants determined by the capacitors 111 a, 111 b, 112 a, 112 b, 113 a and 113 b, the parasitic capacities and the like of the differential amplifiers 101, 102, 103 and 104. Therefore, in a time period T_{cycle }(1), the total delay time T is
T=T1+T2+T3+T4.

[0072]
Next, at time t0′ after a stable time period T_{s }(>T1+T2+T3+T4) has passed, the control circuit 120 turns ON the switches ss1, ss2 and ss3, so that the output voltages V_{c} 1 a, V_{c} 1 b, V_{c} 2 a, V_{c} 2 b, V_{c} 3 a and V_{c} 3 b are initialized or reset to their operating points. Note that the stable time period T_{s }is determined so that the output voltage V_{out }can surely be established.

[0073]
Next, at time t1 after T_{cycle }(1) has passed, the input voltage V_{in }is switched from V_{8 }(1000) to V_{4 }(0100), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Then, the control circuit 120 sequentially turns OFF the switches ss1, ss2 and ss3. In this case, no return delay tires ΔT1, ΔT2 and ΔT3 of FIG. 5 for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a are required. Therefore, in a time period T_{cycle }(2), the total delay time period T is
T=T1+T2+T3+T4.

[0074]
Next, at time t1′ after a stable time period T_{s }has passed, the control circuit 120 turns ON the switches ss1, ss2 and ss3, so that the output voltages V_{c} 1 a, V_{c} 1 b, V_{c} 2 a, V_{c} 2 b, V_{c} 3 a and V_{c} 3 b are initialized or reset to their operating points.

[0075]
Next, at time t2 after T_{cycle }(2) has passed, the input voltage V_{in }is switched from V_{4 }(0100) to V_{6 }(0110), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Then, the control circuit 120 sequentially turns OFF the switches ss1, ss2 and ss3. Even in this case, no return delay times ΔT1, ΔT2 and ΔT3 of FIG. 5 for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a are required. Therefore, in a time period T_{cycle }(3), the total delay time period T is
T=T1+T2+T3+T4.

[0076]
Next, at time t2′ after a stable time period T_{s }has passed, the control circuit 120 turns ON the switches ss1, ss2 and ss3, so that the output voltages V_{c} 1 a, V_{c} 1 b, V_{c} 2 a, V_{c} 2 b, V_{c} 3 a and V_{c} 3 b are initialized or reset to their operating points.

[0077]
Finally, at time t3 after T_{cycle }(3) has passed, the input voltage V_{in }is switched from V_{6 }(0110) to V_{5 }(0101), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Then, the control circuit 120 sequentially turns OFF the switches ss1, ss2 and ss3. Even in this case, no return delay times ΔT1, ΔT2 and ΔT3 of FIG. 5 for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a are required. Therefore, in a time period T_{cycle }(4), the total delay time period T is
T=T1+T2+T3+T4.

[0078]
Thus, in the comparator of FIG. 6, the total delay time is not increased, even when the input voltage V_{in }crosses the reference voltage V_{ref}.

[0079]
In the comparator of FIG. 6, however, in order to surely transmit the transition of the input voltage V_{in }to the output voltage V_{out}, the transition timing of the output voltages of each of the differential amplifiers 101, 102 and 103 is determined by the OFF timings of the switches ss1, ss2 and ss3, respectively. Therefore, since the OFF timings of the switches ss1, ss2 and ss3 must be determined in view of the maximum values of the delay time periods T1, T2, T3 and T4, the stable time period T_{s }is not always short, so that the total delay time T is not always decreased.

[0080]
Also, in the comparator of FIG. 6, in order to amplify a small difference in voltage, each of the fast differential amplifiers 101, 102 and 103 needs to have some amplification, so that the output impedance of each of the fast differential amplifiers 101, 102 and 103 needs to have some value. Therefore, some delay would be generated in rising and falling of signals, so that a transmission delay time is required for the difference in voltage generated by one fast differential amplifier to transmit to the difference in output voltage in the next stage differential amplifier. Thus, it is impossible to decrease the total delay time, which would exhibit a remarkablyadverse effect in the overall high speed comparison operations. Additionally, since the operation speed of the fast amplifiers 101, 102 and 103 per se cannot be increased, the operation speed of the comparator cannot be increased.

[0081]
In FIG. 8, which illustrates a comparator including an embodiment of the amplifier according to the present invention, applied to the sequential successive approximation A/D converter of FIG. 1, differential amplifiers 11, 12, 13 and 14 are connected in series by capacitors 21 a, 21 b, 22 a, 22 b, 23 a and 23 b. The differential amplifiers 11, 12 and 13 according to the present invention are fast/super fast ones of a twoinput and twooutput type with a controllable gain (amplification) and a controllable output impedance operable at a controllable speed, while the differential amplifier 14 is of a twoinput and oneoutput type with a high gain (or amplification) and a high output impedance operable at a low speed. Thus, the entirety of the differential amplifiers 11, 12, 13 and 14 can realize a high gain operable at a high speed.

[0082]
In the fast/super fast differential amplifiers 11, 12 and 13, the relationship between the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a and the output voltages V_{c} 1 b, V_{c} 2 b and V_{c} 3 b is opposite in phase to the relationship between the input voltages V_{1} 1 a, V_{1} 2 a and V_{1} 3 a and the input voltages V_{1} 1 b, V_{1} 2 b and V_{1} 3 b. Also, in the differential amplifier 14, when V_{1} 4 a>V_{1} 4 b, the output voltage V_{out }is high (=V_{DD}), and when V_{1} 4 a≦V_{1} 4 b, the output voltage is low (=GND).

[0083]
The input ends of the differential amplifiers 11, 12, 13 and 14 are connected via switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b to a reference line REF. The switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b are controlled by a control circuit 31 so that the operating points of the input voltages of the differential amplifiers 11, 12, 13 and 14 can be at the reference voltage V_{ref }before the comparison operations.

[0084]
As stated above, the slow differential amplifier 14, which corresponds to the slow differential amplifier 104 of FIGS. 3 and 6, always has a high amplification (gain) and a high output impedance. That is, when the input voltage V_{1} 4 a is lower than the input voltage V_{1} 4 b, the difference therebetween is amplified with a high amplification, so that the output voltage V_{out }is high, for example, V_{DD}. On the other hand; when the input voltage V_{1} 4 a is higher than the input voltage V_{1} 4 b, the difference therebetween is amplified with a low amplification, so that the output voltage V_{out }is low, for example, GND.

[0085]
The control circuit 31 generates an amplification and output impedance control signal CNT and transmits it to the fast/super fast differential amplifiers 11, 12 and 13, so that the amplification and output impedance of the fast/super fast differential amplifiers 11, 12 and 13 are controlled.

[0086]
Note that the comparator of FIG. 8 is integrated into a semiconductor substrate.

[0087]
In FIG. 9, which illustrates a first example of each of the differential amplifiers 11, 12 and 13 of FIG. 8, an amplification and output impedance switching circuit AS1 formed by pchannel MOS transistors Q_{p3}, Q_{p4}, Q_{p5 }and Q_{p6 }is added to the elements of FIG. 4.

[0088]
The pchannel MOS transistors Q_{p3 }and Q_{p4 }forming a first resistance circuit are connected in series between output terminals OUTa and OUTb. The gates of the pchannel MOS transistors Q_{p3 }and Q_{p4 }are connected to the ground terminal GND, while the backgates of the pchannel MOS transistors Q_{p3 }and Q_{p4 }are connected to the power supply terminal V_{DD}. The connection point between the pchannel MOS transistors Q_{p3 }and Q_{p4 }is connected to the gates of the pchannel MOS transistors Q_{p1 }and Q_{p2}. Since the gates of the pchannel MOS transistors Q_{p3 }and Q_{p4 }are grounded, each of the pchannel MOS transistors Q_{p3 }and Q_{p4 }serves as a constant resistance.

[0089]
The pchannel MOS transistors Q_{p5 }and Q_{p6 }forming a second resistance circuit are connected in series between the output terminals OUTa and OUTb. The gates of the pchannel MOS transistors Q_{p5 }and Q_{p6 }receive the amplification and output impedance control signal CNT, while the backgates of the pchannel MOS transistors Q_{p5 }and Q_{p6 }are connected to the power supply terminal V_{DD}. The connection point between the pchannel MOS transistors Q_{p5 }and Q_{p6 }is connected to the gates of the pchannel MOS transistors Q_{p1 }and Q_{p2}. Since the gates of the pchannel MOS transistors Q_{p5 }and Q_{p6 }are controlled by the amplification and output impedance control signal CNT, each of the pchannel MOS transistors Q_{p3 }and Q_{p4 }serves as a variable resistance. For example, when the voltage of the amplification and output impedance control signal CNT is V_{DD}, the variable resistance has a substantially infinite value. Also, when the voltage of the amplification and output impedance control signal CNT is GND, the variable resistance is substantially zero.

[0090]
Note that the size of the pchannel MOS transistor Q_{p3 }is preferably the same as that of the pchannel MOS transistor Q_{p4}, so that the resistance value of the pchannel MOS transistor Q_{p3 }is substantially the same as that of the pchannel MOS transistor Q_{p4}. In this case, the voltages at the gates of the pchannel MOS transistors Q_{p1 }and Q_{p2 }are maintained at the operating points of the output voltages V_{c} 1 a and V_{c} 1 b (V_{c} 2 a and V_{c} 2 b, V_{c} 3 a and V_{C} 3 b). Similarly, the size of the pchannel MOS transistor Q_{p6 }is preferably the same as that of the pchannel MOS transistor Q_{p6}, so that the resistance value of the pchannel MOS transistor Q_{p5 }is substantially the same as that of the pchannel MOS transistor Q_{p6}.

[0091]
The operation of the fast/super fast differential amplifier of FIG. 9 is explained below.

[0092]
When the amplification and output impedance control signal CNT indicates a high voltage such as V_{DD}, the fast/super fast differential amplifier serves as a fast differential amplifier having a first amplification and a first output impedance operable at a fast speed. That is, the pchannel MOS transistors Q_{p5 }and Q_{p6 }are turned OFF, so that the output impedance is determined by a combined resistance of the pchannel MOS transistors Q_{p1}, Q_{p2}, Q_{p3 }and Q_{p4}.

[0093]
On the other hand, when the amplification and output impedance control signal CNT indicates a low voltage such as GND, the fast/super fast differential amplifier serves as a super fast differential amplifier having a second amplification smaller than the first amplification and a second output impedance smaller than the first output impedance operable at a super fast speed. That is, the pchannel MOS transistors Q_{p5 }and Q_{p6 }are turned ON, so that the output impedance is determined by a combined resistance of the pchannel MOS transistors Q_{p1}, Q_{p2}, Q_{p3}, Q_{p4}, Q_{p5 }and Q_{p6}.

[0094]
Thus, when the amplification and output impedance control signal CNT indicates a high voltage, the first output impedance is combined with the next stage capacitance to form a first time constant, so that the difference between the input voltages V_{1} 1 a (V_{1} 2 a, V_{1} 3 a) and V_{1} 1 b (V_{1} 2 b, V_{1} 3 b) is amplified by the first amplification at a fast speed depending upon the first time constant. On the other hand, when the amplification and output impedance control signal CNT indicates a low voltage, the second output impedance is combined with the next stage capacitance to form a second time constant, so that the difference between the input voltages V_{1} 1 a (V_{1} 2 a, V_{1} 3 a) and V_{1} 1 b (V_{1} 2 b, V_{1} 3 b) is amplified by the second amplification at a super fast speed depending upon the second time constant.

[0095]
In summary, if the differential amplifier of FIG. 9 has a low gain (amplification) and a low output impedance operable at a high speed under CNT=V_{DD}, the differential amplifier of FIG. 9 has a much lower gain (much lower amplification) and a much lower output impedance operable at a much higher speed under CNT=GND.

[0096]
The operation of the comparator of FIG. 8 is explained next with reference to FIG. 10. Even in this case, assume that the analog input voltage AIN is between V_{6 }(0101) and V_{6 }(0110) as shown in FIG. 2, and comparison operations are carried out four times with the value of the sequential approximation register 3 of FIG. 1 being 1000, 0100, 0110 and 1010.

[0097]
Before time to, the control circuit 31 turns ON all the switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b, so that the input voltages V_{1} 1 a, V_{1} 1 b, V_{1} 2 a, V_{1} 2 b, V_{1} 3 a, V_{1} 3 b, V_{1} 4 a and V_{1} 4 b of the differential amplifiers 101, 102, 103 and 104 are initialized to V_{ref }(operating point). Also, the control circuit 31 makes the amplification and output impedance control signal CNT high (=V_{DD}), so that the fast/super fast differential amplifiers 11, 12 and 13 are in a fast speed mode. That is, each of the differential amplifiers 11, 12 and 13 serves as a fast differential amplifier having a low gain (amplification) and a low output impedance operable at a fast speed.

[0098]
At time t0, the control circuit 31 turns ON all the switches sf1 a, sf1 b, sf2 a, sf2 b, sf3 a, sf3 b, sf4 a and sf4 b, so that the input voltages V_{1} 1 a, V_{1} 2 a, V_{1} 3 a, and V_{1} 4 a are separated from the input voltages V_{1} 1 b, V_{1} 2 b, V_{1} 3 b, and V_{1} 4 b, respectively. Simultaneously, the control circuit 31 makes the amplification and output impedance control signal CNT low (=GND), so that the fast/super fast differential amplifiers 11, 12 and 13 enter a super fast speed mode defined by a time period TL. That is, each of the fast/super fast differential amplifiers 11, 12 and 13 serves a super fast differential amplifier having a much lower gain (amplification) and a much lower output impedance operable at a super fast speed.

[0099]
Also, at time t0, the input voltage V_{in }is caused to be V_{8 }(1000), so that the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a of the differential amplifiers 11, 12 and 13 are rapidly changed with delay time periods T1 a, T2 a and T3 a, respectively, in accordance with time constants substantially determined by the parasitic capacities and the like within the differential amplifiers 11, 12 and 13. Therefore, when the delay time period T1 a has passed, the output voltages V_{c} 1 a of the differential amplifier 11 is changed to reach a predetermined level. Then, when the delay time period T3 a has passed, the output voltage V_{c} 3 a of the differential amplifier 13 is changed to reach a predetermined level. As a result, the output voltage V_{out }of the slow differential amplifier 14 is changed with a delay time period T4 in accordance with a time constant determined by the parasitic capacitance and the like of the differential amplifier 14. Therefore, after the delay time period T4 has passed, the output voltage V_{out }of the differential amplifier 14 is changed and is finally brought close to V_{DD }or GND at time t1.

[0100]
Next, at time t0′ after the super fast mode time period TL formed by T1 a, T2 a and T3 a (TL>T1 a+T2 a+T3 a) has passed, the control circuit 31 makes the amplification and output impedance control signal CNT high (=V_{DD}), so that the fast/super fast differential amplifiers 11, 12 and 13 return to a fast mode. In this case, the time t0′ is set before time t0″ when the output voltage V_{out }of the slow differential amplifier 14 starts to change.

[0101]
Note that the super fast mode time period TL generally satisfies the following:
TL<T1+T2+T3+ΔT1+ΔT2+ΔT3

[0102]
where T1, T2 and T3 (see: FIG. 5) are delay time periods of the differential amplifiers 11, 12 and 13 serving as fast differential amplifiers; and

[0103]
ΔT1, ΔT2 and ΔT3 (see: FIG. 5) are return delay time periods of the differential amplifiers 11, 12 and 13 serving as fast differential amplifiers. Therefore, in a time period T_{cycle }(1), the total delay time T is
T=T1a+T2a+T3a+T4.

[0104]
Next, at time t1 after T_{cycle }(1) has passed, the input voltage V_{in }is switched from V_{8 }(1000) to V_{4 }(0100), so that the input voltage V_{in }crosses the reference voltage V_{ref}. In this case, the control circuit 31 makes the amplification and output impedance control signal CNT low (=GND), so that the differential amplifiers 11, 12 and 13 enter a super fast mode defined by a super fast time period TL. As a result, no return delay times ΔT1, ΔT2 and ΔT3 of FIG. 5 for the output voltages V_{c1} 1 a, V_{c} 2 a and V_{c} 3 a are required. Therefore, in a time period T_{cycle }(2), the total delay time period T is
T=T1a+T2a+T3a+T4.

[0105]
Next, at time t2 after T_{cycle }(2) has passed, the input voltage V_{in }is switched from V_{4 }(0100) to V_{6 }(0110), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Even in this case, since the differential amplifiers 11, 12 and 13 have entered a super fast mode, no return delay times ΔT1, ΔT2 and ΔT3 of FIG. 5 for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a are required. Therefore, in a time period T_{cycle }(3), the total delay time period T is
T=T1a+T2a+T3a+T4.

[0106]
Finally, at time t3 after T_{cycle }(3) has passed, the input voltage V_{in }is switched from V_{6 }(0110) to V_{5 }(0101), so that the input voltage V_{in }crosses the reference voltage V_{ref}. Even in this case, since the differential amplifiers 11, 12 and 13 have entered a super fast modes no return delay times ΔT1, ΔT2 and ΔT3 of FIG. 5 for the output voltages V_{c} 1 a, V_{c} 2 a and V_{c} 3 a are required. Therefore, in a time period T_{cycle }(4), the total delay time period T is
T=T1a+T2a+T3a+T4.

[0107]
Thus, in the comparator of FIG. 8, the total delay time is not increased, even when the input voltage V_{in }crosses the reference voltage V_{ref}. Also, since the speed of transmission of signals per se is increased by a super fast mode of the differential amplifiers 11, 12 and 13, the speed of transmission of signals can be increased as compared with the second prior art comparator of FIG. 6.

[0108]
In FIG. 11, which illustrates a second example of the fast/super fast differential amplifiers 11, 12 and 13 of FIG. 8, the amplification and output impedance switching circuit AS1 of FIG. 9 is replaced by an amplification and output impedance switching circuit AS2 where the pchannel MOS transistors Q_{p3 }and Q_{p4 }of FIG. 9 are removed. For example, when the voltage of the amplification and output impedance control signal CNT is V_{DD}, each of the pchannel MOS transistors Q_{p5 }and Q_{p6 }has a substantially infinite resistance value. Also, when the voltage of the amplification and output impedance control signal CNT is GND+α where α is a small positive value, each of the pchannel MOS transistors Q_{p6 }and Q_{p6 }has a predetermined resistance value. Thus, the amplification and output impedance switching circuit AS2 is operated in the same way as the amplification and output impedance switching circuit AS1 of FIG. 9.

[0109]
In FIG. 12, which illustrates a third example of the fast/super fast differential amplifiers 11, 12 and 13 of FIG. 8, the amplification and output impedance switching circuit AS2 of FIG. 11 is replaced by an amplification and output impedance switching circuit AS3 where the pchannel MOS transistors Q_{p5 }and Q_{p6 }of FIG. 11 are replaced by variable resistors VR1 and VR2. For example, when the voltage of the amplification and output impedance control signal CNT is V_{DD}, each of the variable resistors VR1 and VR2 has a substantially infinite resistance value. Also, when the voltage of the amplification and output impedance control signal CNT is GND, each of the variable resistors VR1 and VR2 has a predetermined resistance value. Thus, the amplification and output impedance switching circuit AS3 is operated in the same way as the amplification and output impedance switching circuit AS1 of FIG. 9. In FIG. 12, note that the variable resistors VR1 and VR2 have the same value in order to suppress the fluctuation of gate voltages of the pchannel MOS transistors Q_{p1 }and Q_{p2}.

[0110]
Also, the amplification and output impedance switching circuits AS1, AS2 and AS3 of FIG. 9 are replaced by amplification and output impedance switching circuits AS1′, AS2′ and AS3′, respectively, as illustrated in FIGS. 13, 14 and 15, where the conductivity type of MOS transistors is changed. In FIGS. 13 and 14, the logic of the amplification and output impedance control signal CNT is opposite to that in FIGS. 9 and 11. Even the amplification and output impedance switching circuits AS1′, AS2′ and AS3′ of FIGS. 13, 14 and 15 are operated in the same way as the amplification and output impedance switching circuit AS1 of FIG. 9.

[0111]
In FIGS. 9, 11, 12, 13, 14 and 15, note that a plurality of amplifications and a plurality of output impedances can be set by providing a plurality of voltages as the amplification and output impedance control signal CNT.

[0112]
In FIG. 16, which illustrates a modification of the comparator of FIG. 8, the capacitors 21 a, 21 b, 22 a, 22 b, 23 a and 23 b of FIG. 8 are removed, and the amplification and output impedance control signal CNT is supplied from the exterior, not from the control circuit 31. That is, since the differential amplifiers 11, 12 and 13 serve as super fast differential amplifiers with a much lower output impedance in a super fast mode at an initial stage of each comparison operation, the absence of the capacitors 21 a, 21 b, 22 a, 22 b, 23 a and 23 b hardly affects the comparison operation. Also, when the amplification and output impedance control signal CNT is supplied from the exterior, the timing and voltage level of the amplification and output impedance control signal CNT can easily be adjusted, so that these timing and voltage level can be optimized.

[0113]
In FIGS. 8 and 16, all the fast differential amplifiers 11, 12 and 13 are controlled by the amplification and output impedance control signal CNT; however, at least one of the fast differential amplifiers 11, 12 and 13 can be controlled by the amplification and output impedance control signal CNT.